5:15 PM - 6:45 PM
[SVC30-P03] Toward a reconstruction of a total grain-size distribution of a pyroclastic density current: A case study of the 1929 eruption deposit at Hokkaido-Komagatake
Keywords:Hokkaido-Komagatake, Pyroclastic density current, Total grain-size distribution
The pyroclastic density current (PDC) generated by the 1929 eruption of the Hokkaido-Komagatake volcano is a valuable eruption case for validating a numerical simulation model because many physical parameters constraints on the dynamics of the PDC can be obtained. This eruption is the smallest PDC of a collapsing eruption column type compared to other worldwide eruptions (Uesawa et al., 2023). Therefore, it is important to develop more detailed physical parameters of this pyroclastic flow including particle densities (Ishige et al., 2024 in this conference), and thus, we are attempting to calculate the total grain-size distribution (TGSD). To obtain the TGSD of PDC, it is necessary to conduct outcrop-scale grain-size analysis at as many points as possible. We also measured the sediment density for volume-to-mass conversion. In this report, we introduce the methods and results of GSD analysis using heavy equipment and sediment density measurement with 3d geomorphological measurement using LiDAR mounted on the iPhone13pro.
The target area is the southwestern part of the mountain where Murai (1960) calculated the comparable GSD data. The exposures are good in the area, although the area has been modified in the past by surface artificial erosion. The stratigraphic boundary with PDC of the 1640 eruption is also relatively easy to distinguish. We selected four sites (from the upstream, the distances from the vent are 3.6 km, 4.7 km, 5 km, and 5.7 km for points D, C, A, and B, respectively) to provide data spatially balanced collection areas where heavy equipment can be operated. Then, we conducted an outcrop-scale grain size analysis and measured excavated sediment volume with the following procedures.
1. removal of large vegetation.
2. photographing of outcrops with UAV, 3d surveying before excavation, measurement of unit thickness with Leica DISTO D510.
3. confirmation and marking of excavation points.
4. excavation using heavy equipment (hydraulic excavator).
5. separation of coarse particles and sieving of fine particles adhering to coarse particles with brush.
6. sieving by 32 mm sieve and size reduction of sediment less than 32 mm using a custom-made large-size reduction machine.
7. weighing particles of 1024-512 mm, 512-256 mm, 256-128 mm, 128-64 mm, 64-32 mm, and less than 32 mm using an electronic spring weighed. Less than 32 mm samples were measured at twice and six times reduction. Two measurements revealed a bias in the condensation. The bias was corrected by the average reduction rate.
8. post-excavation photography. 3d outcrop survey after excavation using iPhone.
The samples were dried, and the water content was determined for representative coarse grain and matrix samples. The particles smaller than 32 mm were sieved by an electronic sieve and a hand sieve which are intervals of 0.5 φ (φ = -log2d mm; d = mesh size) from 16 mm to 63 μm. The 3d outcrop data measured with the iPhone was aligned using MeshLab and was rewritten into XYZ data and converted into a pseudo-DEM model using QGIS. The volume was calculated from the difference between the pre-and post-excavation raster data.
The GSD and sediment density obtained by the above methods are, from upstream, -1.6, 0.11, -2.69, and 0.18 for the median grain size (Mdφ), 3.61, 3.23, 3.65, and 3.21 for the standard deviation (σφ), and 5.4 m, 1.2 m, 2.9 m, and 1.5 m for the unit thickness at the thickest part, respectively. In comparison with Murai (1960), coarse particles tend to be more abundant in the central part of the PDC deposit in this study. The mean sediment density at the four sites is calculated to be 1,160 kg/m3 with a standard deviation of 70. This value is smaller than the 1,600 kg/m3 assumed by Uesawa et al (2023).
The target area is the southwestern part of the mountain where Murai (1960) calculated the comparable GSD data. The exposures are good in the area, although the area has been modified in the past by surface artificial erosion. The stratigraphic boundary with PDC of the 1640 eruption is also relatively easy to distinguish. We selected four sites (from the upstream, the distances from the vent are 3.6 km, 4.7 km, 5 km, and 5.7 km for points D, C, A, and B, respectively) to provide data spatially balanced collection areas where heavy equipment can be operated. Then, we conducted an outcrop-scale grain size analysis and measured excavated sediment volume with the following procedures.
1. removal of large vegetation.
2. photographing of outcrops with UAV, 3d surveying before excavation, measurement of unit thickness with Leica DISTO D510.
3. confirmation and marking of excavation points.
4. excavation using heavy equipment (hydraulic excavator).
5. separation of coarse particles and sieving of fine particles adhering to coarse particles with brush.
6. sieving by 32 mm sieve and size reduction of sediment less than 32 mm using a custom-made large-size reduction machine.
7. weighing particles of 1024-512 mm, 512-256 mm, 256-128 mm, 128-64 mm, 64-32 mm, and less than 32 mm using an electronic spring weighed. Less than 32 mm samples were measured at twice and six times reduction. Two measurements revealed a bias in the condensation. The bias was corrected by the average reduction rate.
8. post-excavation photography. 3d outcrop survey after excavation using iPhone.
The samples were dried, and the water content was determined for representative coarse grain and matrix samples. The particles smaller than 32 mm were sieved by an electronic sieve and a hand sieve which are intervals of 0.5 φ (φ = -log2d mm; d = mesh size) from 16 mm to 63 μm. The 3d outcrop data measured with the iPhone was aligned using MeshLab and was rewritten into XYZ data and converted into a pseudo-DEM model using QGIS. The volume was calculated from the difference between the pre-and post-excavation raster data.
The GSD and sediment density obtained by the above methods are, from upstream, -1.6, 0.11, -2.69, and 0.18 for the median grain size (Mdφ), 3.61, 3.23, 3.65, and 3.21 for the standard deviation (σφ), and 5.4 m, 1.2 m, 2.9 m, and 1.5 m for the unit thickness at the thickest part, respectively. In comparison with Murai (1960), coarse particles tend to be more abundant in the central part of the PDC deposit in this study. The mean sediment density at the four sites is calculated to be 1,160 kg/m3 with a standard deviation of 70. This value is smaller than the 1,600 kg/m3 assumed by Uesawa et al (2023).